1. Field of the Invention
This invention relates to semiconductor device processing and, in particular, to semiconductor device processing involving etching.
2. Art Background
Semiconductor devices are generally fabricated on a substrate through a series of processing steps including deposition of semiconductor materials, metals, insulators, and the etching of these materials in selected patterns. In one common etching method, a reactive etchant species is generated in a plasma and directed at a high velocity by an electric field towards the material to be etched. Generally during etching, the material being etched is masked in selected regions from this reactive species by a composition such as a polymer (typically denominated a resist) that has been delineated in a desired pattern. Thus, the pattern of the resist is transferred during the etching process to the underlying etched material. As a result, the device configurations desired in semiconductor, insulator, and/or metal regions are produced.
Accelerated reactive species are advantageously utilized in etching because they typically produce anisotropic etching, i.e., etching where the etch rate at any point along the etch pit wall in the direction of etchant species momentum is at least ten times greater than the etch rate in the direction normal to this velocity vector. Two effects generally contribute to the attainment of anisotropic etching. In one circumstance, a material to be etched, such as silicon, reacts with certain species, e.g., Cl.sup.+, only when the species has significantly higher momentum than that associated with room temperature. Thus, when such a species is accelerated essentially perpendicularly to the substrate surface, etching preferentially occurs in this direction rather than the remaining random directions associated with etchant species which are only thermally activated. In a second circumstance, such as in the etching of aluminum by species generated in a C1.sub.2 /C.sub.2 F.sub.6 plasma, material which is resistant to isotropic etching is produced during the etching process, is redeposited on the sidewalls of the etch void, and promotes anisotropic etching. (See U.S. Pat. No. 4,208,241, issued June 17, 1980, where a recombinant species forms such a resistant material.) That is, the redeposited material acts as an etch mask.
The attainment of anisotropic etching through one or both of these mechanisms is significant because it promotes maintenance of the unetched region dimensions (frequently called linewidth control), facilitates subsequent processing, and allows lines to be closely spaced. For example, when isotropic etching, i.e., a lateral etch rate that is at least one-tenth of the etch rate in the etchant momentum direction, rather than anisotropic etching is achieved, etch profiles, 14, such as those shown in FIG. 1 are produced utilizing a resist, 15. Obviously, the attainment of isotropic rather than anisotropic etching reduces linewidth control, i.e., the percentage between 1) the largest deviation of any feature from the corresponding desired feature, 12, defined by the mask at the substrate surface and 2) one-half the linewidth, 12, defined by the mask. (A feature deviation is thus the distance from any point on an etch sidewall measured perpendicularly to a surface extending from the extremities of the mask region defining this feature in a direction perpendicular to the surface at each point along the mask extremity.) Similarly, linewidth control is also compromised, as shown in FIG. 2 where 15 indicates the resist, if the sidewalls slope in the opposite direction. Configurations such as those shown in FIGS. 1 and 2 are undesirable because they interfere with subsequent processing as well as contributing to loss of linewidth control. For example, it the etched line is itself to be used as a mask, e.g., for an ion implantation mask, any linewidth gain or loss augments the associated inaccuracies in subsequent mask use. Alternatively, if an insulating layer is to be formed on the etch sidewalls by deposition onto all surfaces followed by an anisotropic etch to remove the material from lateral surfaces, then the tapered profile in FIGS. 1 and 2 will compromise the insulating layer on the sidewall during the anisotropic etch.
Although sidewall redeposition during etching has been associated in many circumstances with the maintenance of anisotropic etching, it does present some difficulties. As discussed by Kinsbron, Levinstein, and Willenbrock in U.S. Pat. No. 4,343,677, issued Aug. 10, 1982, it is desirable to remove these sidewall redeposits because (1) they tend to be dislodged during subsequent processing, (2) they often assume undesirable shapes, and (3) they frequently have undesirable electrical or mechanical properties. Sidewall redeposition also does not always ensure anisotropy. Despite the occurrence of sidewall redeposition and the use of energetic reactive species, anomalous etching patterns such as shown at 16 in FIG. 3 (where 15 is the resist) have been observed. These anomalies are generally undesirable because they degrade feature mechanical stability and conductivity.